1. Field of the Invention
This invention is concerned with a process for the manufacture of lubricating oils. In particular, it is concerned with a particular combination and sequence of catalytic unit processes whereby a hydrocracked lube oil having a low pour point and a high viscosity index is produced.
2. Prior Art
Refining suitable petroleum crude oils to obtain a variety of lubricating oils which function effectively in diverse environments has become a highly developed and complex art.
In general, the basic notion in conventional lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant stock having a predetermined set of properties such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant stock consists of a set of subtractive unit operations which removes the unwanted components. The most important of these unit operations include distillation, solvent refining, and dewaxing, which basically are physical separation processes in the sense that recombination of all the separated fractions would reconstitute the crude oil.
Unfortunately, crude oils suitable for the manufacture of lubes by conventional processing are becoming less available due to exhaustion of reserves, and the reliability of a steady, adequate supply from a known source is a matter of concern due to political instability.
The desirability of upgrading a crude fraction normally considered unsuitable for lubricant manufacture to one from which good yields of lubes can be obtained has long been recognized. The so-called "hydrocracking process" has been proposed to accomplish such upgrading. In this process, a suitable fraction of a poor grade crude, such as a California crude, is catalytically reacted with hydrogen under pressure. The process is complex in that some of the oil is reduced in molecular weight and made unsuitable for lubes, but concurrently a substantial fraction of the polynuclear aromatics is hydrogenated and cracked to form naphthenes and isoparaffins. The catalyst and the process conditions usually are selected to provide an optimal conversion of the polynuclear aromatic content of the stock, since it is primarily this component that degrades the viscosity index of the stock.
The hydrocracking process for increasing the availability of lube oils has an attractive feature that is not immediately apparent. Generally, the composition and properties of hydrocracked stocks are not particularly affected by the source and nature of the crude, i.e. they tend to be much more alike than lube fractions prepared from different crudes by conventional means. Thus, the hydrocracking process promises to free the refiner from dependence on a particular crude, with all of the advantages that this freedom implies.
Hydrocracked lubricating oils generally have an unacceptably high pour point and require dewaxing. Solvent dewaxing is a well-known and effective process but expensive. More recently catalytic methods for dewaxing have been proposed. U.S. Pat. No. Re. 28,398 to Chen et al, herein incorporated by reference, describes a catalytic dewaxing process wherein a particular crystalline zeolite is used.
Hydrofinishing processes have been successful in replacing clay decolorization. In such processes, color bodies and other undesirable sulfur and nitrogen compounds are chemically transformed in the presence of hydrogen with essentially 100 percent recovery of the charge oil as finished lube stock. A modification of the hydrofinishing process has been proposed in U.S. Pat. No. 4,162,962 to Stangeland, and the process adapted to hydrogenating unstable hydrocracked lube oils. The entire content of this patent is incorporated herein by reference.
In general, whether conventional or catalytic processes or combinations of these are used or are proposed to prepare high viscosity index (hereinafter denoted "high V.I.") lubes of low pour point, the process scheme usually contemplated is to remove or to convert to isoparaffins the undesirable polynuclear aromatic hydrocarbons prior to separation of the waxes. U.S. Pat. No. 3,755,145 to Orkin describes a process for catalytic hydrocracking of waxy raw distillates and residual stocks with a catalyst mixture comprising a hydrogenation component and at least two separate acidic cracking catalysts, one of which is a crystalline aluminosilicate of the ZSM-5 type. In this process it appears that dewaxing and conversion of polynuclear aromatics occurs simultaneously.